FUNDAMENTALS OF MICROBIOLOGY IN RELATION TO CLEANSING 51 grow, and any present will die out. Yeasts and moulds are a significant exception, and the few pathogenic species (e.g. Candida albicans) are much more resistant to acids than bacteria. Thus C. albicans grows best over the range 5.1-t3.4 and can grow over a wide pH range (2). This acidity factor is of special interest to cosmetic chemists because the secretions of the skin are acid, being usually about pH 5.4. Moreover the free fatty acids may exercise a germicidal effect per se. This natural protective effect probably plays an important role in protecting the skin against invading organisms. These can do little harm against the intact skin but may invade if the skin is damaged. In extreme cases the pH of the skin may be as low as 4, and this secretory mechanism has been called the 'acid mantle' of the skin. It follows that frequent washing with a strongly alkaline soap is not to be recommended, and prolonged soaking in hot baths is also inadvisable for the same reason. Scientifically the skin should be cleansed with a preparation at about pH 5.4 rather than with an alkaline material. Basically it is always sounder to assist and stimulate the body's natural protective mechanism rather than try and kill bacteria by 'disin- fectants' and other means. There will always be the occasion when a virulent organism will get through. To be really effective an antiseptic must have strong biocidal properties, and unfortunately there is usually a good correlation between biocidal power and traumatic effect on animal tissue. Contemporary medical thought is veering in this direction. For example, it is now the practice not to remove the natural protective material (vernix caseosa) on the new born infant and wash it by orthodox means (alkaline soap) immediately after birth. The warm water and soap cleansing is left over for a few days, thus allowing the natural mechanisms of the skin to protect the infant against any fortuitous contamination in early extra- uterine existence. The other major example of a natural acid protective mechanism is the acidity of the stomach. In healthy adults the pH of the active gastric secretion is about 2, at which no ordinary pathogens can survive. Sub-acid secretions (pH 2-5) allow acid-resistant types such as Str. salivarius to pass through into the bowel, and in cases of gross acid deficiency (pH 5) most pathogens can survive easily. This is the main reason why young infants and very old people are much more susceptible to food poisoning or infective enteriris than healthy adults. In general young infants and old people are much more susceptible to infections of any type. The young have not acquired much immunity and in old people there is a lowered efficiency of all biochemical mechanisms in the body.

52 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The quantitative approach-the 'communal phenomenon' All studies and control tests in bacteriology should be quantitative in other words an attempt should always be made to estimate the number in- volved, although the error may be large. The reason is that the onset of an infection, or the development of a microbiological defect in any product, is dependent on the initial number of organisms concerned. Apart from a few exceptions, it is doubtful whether a single bacterial cell ever did anyone any harm. It appears to be necessary for at least a few organisms to establish themselves and adapt their environment for growth (the 'communal effect'). This is clearly illustrated by figures for the minimum infectiw dost for well recognized diseases (Table I). It follows that any reduction in numbers of micro-organisms is worthwhile, even if a complete kill is im- possible for practical reasons. Table I. Minimum infectlye doses (approx.) for pathogens in human beings Disease Number of cells Typhoid fever 3 Tuberculosis 100 Cutaneous moniliasis 100 000 Salmonelloses 100 000 to 1 000 000 (other than typhoid fevers) Pseudomonas aeruginosa This organism, also called Ps. pyocyanea and the 'green pus organism', has assumed considerable importance in medicine, surgery, pharmaceutics and cosmetic preparations in recent years. Not only is it capable of being a virulent pathogen but it is so resistant to commonly used disinfectants and antiseptic preparations that it can often be isolated from them in start- lingly large numbers. It is now being found in almost all watery environments in hospitals, nurseries, kitchens and other places where food is prepared and human beings work. Many interesting and disturbing examples are continually coming to light. Quite recently severe cases of mastitis in a valuable dairy herd were found to be caused by a Pseudomonas aeruginosa contamination of the water used for washing the cows' udder. This was ultimately traced to a dead rat